This invention is generally directed to transmitters having a linear final power amplifier (PA) and more specifically directed to such amplifiers which are corrected for amplitude or phase distortion by predistorting the input signal to the PA.
Linear amplifiers are needed when nonconstant envelope amplitude modulated signals are to be amplified such as in a single sideband transmitter. Even "linear" amplifiers do not provide an output signal which is an amplified exact duplicate of the input signal with regard to magnitude and phase.
In an effort to correct for such nonlinearities, circuits have been devised which "predistort" the input signal to a PA to take into account the nonlinearities of the PA so that its output has increased linearity. Typically the PA output signal is sampled and utilized as a feedback signal by such circuitry to generate the predistorted signal.
Since the PA must be operating in order to generate the feedback signal, problems can occur with regard to off-frequency or other unauthorized transmissions should a phase locked loop (PLL) in the predistortion circuit become unlocked. A temporary unlocked condition could be caused by a change in operating frequency or a transient condition that affects the predistortion circuit or the PA.